Frequent Questions

What is the "Highest 5"?

The highest 5 on the airnow.gov website is a computer-generated list of the highest five Air Quality Index (AQI) forecasts for the current and next day, along with a list of the five cities with the highest current AQI.

Here’s how it works:

For forecasts: Every day, state and local air agencies submit local AQI forecasts to EPA, which then makes them available on the airnow.gov website. The forecasts generally are updated daily, but a state or local agency can update them more frequently if needed. The AirNow system generates a list of the highest five forecasts, which appear on the airnow.gov website.

For current conditions: Ozone and particle pollution monitors across the country report current air quality readings to AirNow around the clock. The AirNow system generates a list of the five areas where the AQI for ozone or particle pollution is currently the highest at any given hour; this list automatically updates hourly, so the list of cities on the airnow.gov website with the “Highest 5” current conditions could change several times in a single day.

About the AQI:

The Air Quality Index is EPA’s color-coded tool for communicating air quality to the public. It tells you how clean or polluted your air is, and what associated health effects might be a concern for you. Cities of 350,000 must report the daily AQI for ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide and nitrogen dioxide.

As a public service, state and local air agencies also issue AQI forecasts for ozone and particle pollution – two of the pollutants most common across the country. State and local agencies provide ozone forecasts for 403 cities, and particle pollution forecasts for 423 cities. EPA makes these forecasts broadly available through the airnow.gov website, app, and AirNow EnviroFlash.